Such compressors, which may pump relatively large amounts of air with little compression, have an axially reciprocating piston carrying an armature which is moved through a magnetic field generated by one or more coils. The return stroke of the piston is typically caused by a compression spring. There are many examples of such compressors in the prior art, and reference is made to the following patent specifications as examples, particularly of interest in relation to the present invention:
GB-A-1529597, GB-A-2041092, GB-A-2206931, U.S. Pat. Nos. 4,090,816, 4,718,832, 4,867,656 and 5,100,304.
Such compressors are required to operate without lubrication and with as little maintenance as possible, for long periods. Their typical rate of operation is the mains frequency, e.g. 50 Hz or 60 Hz. Consequently, the compressor must be designed so that wear caused by the moving piston is minimized, so as to avoid the creation of leakage paths which lead to inefficiency. Another problem is noise, particularly where the compressor is to be used in a quiet environment such as a hospital. Noise may arise as a result of wear of the piston or a surface which it contacts, or from contact of the piston with the cylinder head.
It is common for the compressor to have a piston guide mounted in its body, on the opposite side of the driving coil from the cylinder in which the piston head moves. The piston carries a guide piston, which slides axially in the piston guide and thus radially locates and guides the rear end of the piston. Such a piston guide is shown for example in U.S. Pat. Nos. 4,090,816, 4,718,832 and GB-A-1529597 mentioned above. A piston guide integral with the body casing is shown in GB-A-2206931. An alternative form of piston guide is a rod extending through the centre of the piston, as shown for example in U.S. Pat. No. 5,100,304 and GB-A-2041092.
For efficiency of the electromagnetic drive, the piston guide and the metal casing of the compressor must be electrically isolated. This may involve the use of insulating bushings around screws used to attach the piston guide. It is known to anodize the surface of the piston guide, in order to assist such electric isolation (GB-A-1529597).
To ensure highly precise location of the piston, it is important that the piston guide is very accurately located relative to the axis of the cylinder. It is known to provide a registration surface on the piston guide, particularly an outer peripheral cylindrical surface, which is fitted into an internal cylindrical surface of a body part of the pump. This is shown for example in GB-A-1529597, U.S. Pat. Nos. 4,718,832 and 4,090,816, in which the cylinder of the compressor is provided by a cylindrical insert having an exterior peripheral surface on another surface of the body having a different diameter from that receiving the piston guide. GB-A-2206931 shows a similar arrangement, in which the piston guide is an insert in a sleeve integral with a rear body part, while the cylinder surface is provided by an insert in the front body part. The two inserts are of different external diameter.
The present inventors have found that constructions of this type provide insufficiently accurate alignment of the piston guide, relative to the cylinder, leading to uneven wear of the piston during use.